A powered surgical instrument, surgical tool, used with some frequency is the powered surgical saw. This type of instrument is used to remove tissue, including bone and cartilage. Attached to the saw is a saw blade. A drive assembly internal to the saw reciprocates the blade in a back and forth motion. Often the saw includes a moving head. The head is the component of the saw to which the blade is mounted. Some blades are constructed to pivot back and forth, oscillate, in the plane in which the blade is oriented. This type of blade is referred to as a sagittal saw blade. A sagittal saw blade is provided with teeth that extend forward from the distal end of the blade body.
Many sagittal saws and their complementary blades are designed so that the blade extends distally forward of the blade head. One such assembly is disclosed in the Applicant's U.S. Pat. No. 8,100,912/PCT Pub. No. WO 2007/011542, the contents of which are incorporated herein by reference. This type of saw and blade is used to remove a section of bone. This is the most common type of sagittal saw.
A species of the sagittal saw is the acetabular cup remover. As implied by its name, this type of surgical power instrument is used to remove an artificial acetabular cup. An artificial acetabular cup is sometimes implanted in a patient during hip replacement surgery. Hip replacement surgery consists of the replacement of the existing ball and socket of the hip joint with prosthetic replacements. The head of the femur, the ball, is typically removed and replaced with a femoral component made of biocompatible material. This component mirrors the structure of the original bone. The acetabulum, the socket in the hip, is often reamed to form a hole. An artificial acetabular cup that corresponds and cooperates with the femoral component is fitted in the hole. This artificial acetabular cup often includes an outer shell constructed of a metallic material. Typically this shell is in the form of a hollowed out semi-sphere. An insert, constructed of plastic, ceramic or metal, is seated in the outer shell. In many cases, the acetabular cup component is anchored in the bone with cement. Some cups are press fit in place. Still other cups are held in place by screws or fastening tabs integral with the cup itself. A combination of these fastening methods may be employed. Sometimes, owing to the shape of the outer shell and/or the application of compound that enhance bone growth, the outer shell is designed to foster the growth of bone adjacent the shell. This new bone anchors the cup to the rest of the hip. Total hip replacement surgery has often proven successful in relieving many problems associated with the hip joint.
Total hip replacement surgery is often successful. Nevertheless, it is sometimes necessary to perform the same surgery on the same hip. This may be necessary in situations in which wear or infection degrade the performance of the installed cup and femoral head. This sub-set of total hip replacement surgery is sometimes called revision surgery. In a revision surgery, it is necessary to remove the acetabular cup previously implanted in the hip. As mentioned above, these components may have been cemented in place or otherwise held by bone or fibrous tissue that may have grown in and around the component. Thus, their removal requires the cutting or chipping away of cement or bone material.
The acetabular cup remover is employed to remove the artificial acetabular cup. This type of saw includes a pivot head that seats in the socket of the cup. A saw head is connected to the pivot head. The complementary blade has a foot that projects outwardly from the head. The blade has a trunk, a main body, that, relative to the foot, extends downwardly. More particularly, the trunk is shaped to curve around the outer surface of the cup the saw is employed to remove. Teeth extend outwardly from the end and sometimes the side of the trunk. When this type of saw is actuated, the saw head is oscillated to cause a similar back and forth motion of the blade. The blade is then driven around the outer surface of the cup. As a consequence of the oscillatory movement and the blade being simultaneously driven around the cup, the blades, cuts, chips away, bone and any cement that holds the cup to the bone. PCT Pub. No. WO 2014/133536/US Pat. Pub. No. US 2015/0359641 A and PCT Pub. No. WO 2014/207552/US Pat. Pub. No. 2016/0100846 A1, both of which are explicitly incorporated herein by reference, disclose an acetabular cup remover and a number of different blades that can be used with this type of saw.
A surgical sagittal includes an assembly for removably holding the blade to the saw head. This is because the blade, to ensure sterility is removed and replaced between surgeries on different patients. Often a new blade is used for each surgery. This is because upon use of the blade, the teeth are immediately dulled. Owing to the economics, it is often more cost effective to use a new blade with each patient than go to the expense of sterilizing and resharpening previously used blade. This means that each time a saw or acetabular cup remover is used for a new patient, a new blade is fitted to the saw. Moreover, during the process of removing acetabular cup, often two or more blades are used in sequence. The first blade has a trunk that subtends a first arc. This blade is used to form an initial shallow cut around the acetabular cup being removed. Once this shallow cut is formed, a second blade is fitted to the acetabular cup remover. This second blade subtends an arc greater than that subtended by the first blade. The second blade is used to form a deeper cut. This deeper cut typically extends completely around the cup. Once this deep cut is formed, the cup is then removed from the hip.
A surgical sagittal, include a saw designed as an acetabular cup remover, is typically designed so that the head has a slot. The slot is the void space dimensioned to receive the proximal end of the blade. Often the proximal end of the blade is provided with one or more openings. Each opening is dimensioned to receive a pin that is moveably mounted to the saw head. The seating of the pin the blade opening releasably holds the blade to the head.
It is common practice to collectively dimension the saw head and blade so the slot allows the close slip fitting of the blade in the slot. This slip fit dimensioning facilitates the relatively easy insertion of the blade into and removal of the blade from the saw head. An inevitable amount of this component dimensioning is that within the slot, there is small clearance between the blade and the interior surfaces of the saw head that define the slot. This means that within the slot the blade has some space to move.
Owing to this tendency of the blade to move within the slot, the back-and-forth movement of the blade is not always in phase with the back-and-forth movement of the saw head. More specifically this occurs because when the saw head reverses direction, owing to the blade having a momentum in the opposite direction, the blade continues to move in the first direction. Thus, there may be times in the movement of the saw head and blade where these two components move in the opposed directions. This can result in the blade striking an adjacent surface of the saw head. This action is sometimes referred to as blade slap. A result of blades continually slapping against the saw head is that the material forming the head can fatigue. This component fatigue can result in the fracturing of the saw head. Once such a fracture occurs, at a minimum, it is necessary to replace the saw head.
This movement of the blade relative to the saw head can also result in the blade moving to a less than optimal position for the procedure being performed. When a conventional sagittal saw is used, the blade is often placed in a resection guide. A resection guide is a block that is affixed to the bone adjacent where the cut is to be formed. The block is formed with one or more slots. The slots serve as guide paths through which the saw blade is inserted. By cutting the bone along the guide paths defined by the slots, the surgeon shapes the remaining bone to have a selected, precisely defined shape. This precision shaping of the bone is often performed to ensure the proper fitting of an orthopedic implant to the bone. Owing to the flexure of the blade when fitted in one of these slots, the blade can gall, wear the material that defines the slots. This can result in the shape of the slot deforming from the shape needed to ensure that a cut formed based on the shape of the slot has the desired shape. Once this deformation of the resection guide occurs, the guide is no longer useful.
This blade shifting is also disadvantageous when it occurs relative to the head of an acetabular cup remover. This is because the acetabular cup remover is designed so that when the blade is fitted to the saw, and the blade moved around the cup being removed, the blade ideally should move as close as possible around the cup without pressing against the cup. If the blade is spaced away from the cup, more bone is removed from the patient than is necessary to remove the acetabular cup. This can complicate the process of fitting the replacement cup. If the blade presses against the acetabular cup the metal-against-metal movement of the blade, at a minimum, reduces the efficiency of the bone removal process. A more adverse result from this contact is the deformation of the blade or the cup. Either of these events can appreciably complicate the removal of the old cup and the fitting of the replacement cup.
There is therefore a desire to provide a surgical sagittal saw and complementary blade that are constructed so that, when the saw and blade are actuated, the saw head and blade move as a single rigid structure. A means to satisfy this design objective is to design the saw head and blade so the blade is compression fit in the saw head. This can be impracticable because it can require the use of force to first insert the blade into the saw head and then to remove the blade from the head. Having to employ this kind of force can slow the blade removal and insertion processes. Further, the teeth of these blades are very sharp. To reduce the likelihood that the individual tasked with blade insertion and blade removal inadvertently cut himself/herself on the teeth, it might be desirable for this individual task to use a tool to apply the force. The need to provide this type of force with a tool can further complicate the processes of inserting and removing the blades.
An alternative means to ensure the saw head and blade move as a single rigid body is to provide a clamping assembly that, when set, applies an appreciable amount of force to the blade to hold the blade to the saw head. This typically means that the individual charged with blade insertion and blade removal apply a significant amount of force to reset and release the clamping assembly. Requiring the individual responsible to perform these tasks to apply these forces can complicate the process of inserting and removing the blade. This is especially true if the individual has limited arm and hand strength. Further, if these forces are not properly applied, especially the force required to set the clamping assembly, the blade may not be fully locked to the saw head. When the saw is actuated this could result in a clearly undesirable event, the blade working free from the saw.